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| United States Patent |
6,021,359
|
|
Sakakibara
, et al.
|
February 1, 2000
|
Method and apparatus for determining an inspection schedule for a
production line
Abstract
In a check schedule decision method, after simulating the production line
to produce a time-varying number of in-process workpieces in a designated
process unit, a check-available time period for the designated process
unit is detected based on a comparison of the time-varying number of
in-process workpieces to a predetermined level. Further, the production
line is simulated under a constraint that the designated process unit is
stopped operating during the check-available time period to produce a
simulated output of the production line being simulated. When the
simulated output is not smaller than a predetermined target figure, a
check schedule of the designated process unit is decided within the
check-available time period.
| Inventors:
|
Sakakibara; Takako (Yamagata, JP);
Togashi; Yoichi (Yamagata, JP);
Muraoka; Yukihiro (Yamagata, JP)
|
| Assignee:
|
NEC Corporation (JP)
|
| Appl. No.:
|
920466 |
| Filed:
|
August 29, 1997 |
Foreign Application Priority Data
| Current U.S. Class: |
700/108; 700/105; 700/109; 700/110 |
| Intern'l Class: |
G06F 019/00; G06G 007/66 |
| Field of Search: |
364/468.15,468.06,474.04,474.15,474.16,474.17
219/69.17
|
References Cited
U.S. Patent Documents
| 5008842 | Apr., 1991 | Nagai et al. | 364/551.
|
| 5255181 | Oct., 1993 | Chapman et al. | 364/401.
|
| 5539652 | Jul., 1996 | Tegethoff | 364/490.
|
| 5691906 | Nov., 1997 | Togashi et al. | 364/468.
|
Primary Examiner: Grant; William
Assistant Examiner: Robinson; Victoria
Attorney, Agent or Firm: Ostrolenk, Faber, Gerb & Soffen, LLP
Claims
What is claimed is:
1. A method for deciding an inspection schedule of at least one process
unit included in a production line, comprising the steps of:
a) simulating the production line to produce a time-varying number of
in-process workpieces in a designated process unit;
b) detecting an available checking time period for the designated process
unit based on a comparison of the time-varying number of in-process
workpieces versus a predetermined level, and based on a predetermined
checking time required to check the designated process unit;
c) simulating the production line under a constraint that the designated
process unit is stopped operating during the available checking time
period to produce a simulated output of the production line being
simulated; and
d) deciding an available checking schedule of the designated process unit
within the available checking time period when the simulated output is not
smaller than a predetermined target figure.
2. The method according to claim 1, wherein the step b) comprises:
comparing the time-varying number of in-process workpieces versus the
predetermined level;
detecting a time period during which the time-varying number of in-process
workpieces is not greater than the predetermined level; and
determining the time period as the available checking time period when the
time period is not shorter than said predetermined checking time required
to check the designated process unit.
3. The method according to claim 1, wherein
the step b) detects a plurality of available checking time periods for the
designated process unit;
the step c) simulates the production line under a constraint that the
designated process unit is stopped operating during one selected from the
plurality of available checking time periods to produce a simulated output
of the production line being simulated; and
the step d) decides a checking schedule of the designated process unit
corresponding to the selected one of the available checking time periods
when the simulated output is not smaller than the predetermined target
figure.
4. The method according to claim 3, further comprising the steps of:
e) repeating the steps c) and d) while sequentially selecting the available
checking time periods until the simulated output obtained by the step c)
is not smaller than the predetermined target figure.
5. The method according to claim 3, wherein the step b) comprises:
comparing the time-varying number of in-process workpieces versus the
predetermined level;
detecting a plurality of time periods during each of which the time-varying
number of in-process workpieces is not greater than the predetermined
level; and
determining a time period as the available checking time period when the
time period is not shorter than a predetermined checking time required to
check the designated process unit.
6. The method according to claim 5, wherein the available checking time
period is determined by comparing the time period to the predetermined
checking time while sequentially selecting the time period from the
plurality of time periods.
7. The method according to claim 1, further comprising the steps of:
extending a simulation period during which the step a) is performed when an
available checking time period is not detected in the step b); and
repeating the step b) until at least one available checking time period is
detected.
8. The method according to claim 1, wherein, in the step b), the
predetermined level is 10% an average of number of in-process workpieces a
day in the designated process unit.
9. The method according to claim 1, wherein steps a) to d) are performed by
a computer under control of a program stored in a memory of the computer.
10. A method for deciding an inspection schedule of a plurality of process
units included in a production line, comprising the steps of:
a) simulating the production line to produce a time-varying number of
in-process workpieces in each of the process units;
b) detecting an available checking time period based on a comparison of the
time-varying of in-process workpieces versus a predetermined level for
each process unit, and based on a predetermined checking time required to
check the designated process unit;
c) simulating the production line under a constraint that each process unit
is stopped operating during the available checking time period therefor to
produce a simulated output of the production line being simulated; and
d) deciding an available checking schedule of each process unit within the
available checking time period therefor when the simulated output is not
smaller than a predetermined target figure.
11. The method according to claim 10, wherein the step b) comprises:
comparing a time-varying number of in-process workpieces versus a
predetermined level for each process unit;
detecting a time period during which the time-varying number of in-process
workpieces is not greater than the predetermined level; and
determining the time period as the available checking time period when the
time period is not shorter than said predetermined checking time required
to check the process unit.
12. The method according to claim 10, wherein
the step b) detects a plurality of available checking time periods for each
process unit;
the step c) simulates the production line under a constraint that each
process unit is stopped operating during one selected from the available
checking time periods to produce a simulated output of the production line
being simulated; and
the step d) decides an available checking schedule of each process unit
within the selected one of the available checking time periods when the
simulated output is not smaller than the predetermined target figure.
13. The method according to claim 12, further comprising the steps of:
e) repeating the steps c) and d) while sequentially selecting the available
checking time periods until the simulated output obtained by the step c)
is not smaller than the predetermined target figure.
14. The method according to claim 12, wherein the step b) comprises:
comparing a time-varying number of in-process workpieces to a predetermined
level in each process unit;
detecting a plurality of time periods during each of which the time-varying
number of in-process workpieces is not greater than the predetermined
level; and
determining a time period as an available checking time period when the
time period is not shorter than the predetermined checking time.
15. The method according to claim 14, wherein the available checking time
period is determined by comparing the time period to the predetermined
checking time while sequentially selecting the time period from the time
periods.
16. The method according to claim 10, further comprising the steps of:
extending a simulation period during which the step a) is performed when an
available checking time period is not detected in the step b); and
repeating the step b) until at least one available checking time period is
detected.
17. The method according to claim 10, wherein, in the step b), the
predetermined level is 10% an average of number of in-process workpieces a
day in each process unit.
18. The method according to claim 10, wherein steps a) to d) are performed
by a computer under control of a program stored in a memory of the
computer.
19. An inspection scheduler for at least one process unit included in a
production line, comprising:
a simulator for simulating the production line under a constraint to
produce a time-varying number of in-process workpieces in a designated
process unit and a simulated output of the production line being
simulated;
a detector for detecting an available checking time period for a designated
process unit based on a comparison of the time-varying number of
in-process workpieces versus a predetermined level, and based on a
predetermined checking time required to check the designated process unit;
and
a controller for controlling the simulator such that the simulator
simulates the production line under a constraint that the designated
process unit is stopped operating during the available checking time
period to produce a simulated output of the production line, and for
deciding a checking schedule of the designated process unit within the
available checking time period when the simulated output is not smaller
than a predetermined target figure.
20. An inspection scheduler for a plurality of process units included in a
production line, comprising:
a simulator for simulating the production line under a constraint to
produce a time-varying number of in-process workpieces in each of the
process units and a simulated output of the production line being
simulated;
a detector for detecting an available checking time period based on a
comparison of the time-varying number of in-process workpieces versus a
predetermined level for each process unit, and based on a predetermined
checking time required to check the designated process unit; and
a controller for controlling the simulator such that the simulator
stimulates the production line under a constraint that each process unit
is stopped operating during the available checking time period to produce
a simulated output of the production line and for deciding a checking
schedule of each process unit within the available checking time period
when the simulated output is not smaller than a predetermined target
figure.
21. A storage medium storing a program for causing a computer to decide an
inspection schedule of at least one process unit included in a production
line, by carrying out the steps of:
a) simulating the production line to produce a time-varying number of
in-process workpieces in a designated process unit;
b) detecting an available checking time period for the designated process
unit based on a comparison of the time-varying number of in-process
workpieces versus a predetermined level, and based on a predetermined
checking time required to check the designated process unit;
c) simulating the production line under a constraint that the designated
process unit is stopped operating during the available checking time
period to produce a simulated output of the production line being
simulated; and
d) deciding an available checking schedule of the designated process unit
within the available checking time period when the simulated output is not
smaller than a predetermined target figure.
22. A storage medium storing a program for causing a computer to decide an
inspection schedule of a plurality of process units included in a
production line, by carrying out the steps of:
a) simulating the production line to produce a time-varying number of
in-process workpieces in each of the process units;
b) detecting an available checking time period based on a comparison of the
time-varying of in-process workpieces versus a predetermined level for
each process unit, and based on a predetermined checking time required to
check the designated process unit;
c) simulating the production line under a constraint that each process unit
is stopped operating during the available checking time period therefor to
produce a simulated output of the production line being simulated; and
d) deciding an available checking schedule of each process unit within the
available checking time period therefor when the simulated output, is not
smaller than a predetermined target figure.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to management of a production line
including a plurality of process units, and in particular to a method for
deciding an inspection schedule of the process units.
2. Description of the Related Art
A production line having a plurality of process units arranged in series
must be periodically checked for proper and stable operation. Since a
process unit must stop operating in order to be checked, it is very
important to decide upon an inspection schedule of the process units so
that an output of the production line is not decreased by a large amount.
Conventionally, a simulation technique is used to decide such upon an on
schedule. More specifically, a model of the production line is created by
adding many constraints and then a managing operator explores the model's
behavior displayed on screen, that is, a change of the number of
in-process workpieces in the simulated production line. Based on the
time-varying number of in-process workpieces, the managing operator
decides the inspection schedule of the process units.
However, such a model is only as good as its input constraint data. If the
input constraint data is not correct, the model does not accurately mimic
the behavior of the real-world production line being simulated. In the
case where the simulation is performed without consideration of the speed
of workpieces conveyed, for example, the time-varying number of in-process
workpieces substantially departs from that of the real-world production
line, resulting in reduced reliability in line control.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method which decides a
checking schedule so as to provide reliable and stable operation of a
production line even while some process units are being checked.
Another object of the present invention is to provide a method which
determine a checking schedule so as to provide an acceptable output while
maintaining an acceptable number of in-process workpieces in a production
line even while some process units are being checked.
According to the present invention, after simulating the production line to
produce a time-varying number of in-process workpieces in a designated
process unit, an available checking time period for the designated process
unit is detected based on a comparison of the time-varying number of
in-process workpieces versus a predetermined level. Further, the
production line is simulated under a constraint that the designated
process unit is stopped operating during the available checking time
period to produce a simulated output of the production line being
simulated. When the simulated output is not smaller than a predetermined
target figure, a check schedule of the designated process unit is decided
within the available checking time period.
In the case where a plurality of available checking time periods are
detected for the designated process unit, it is preferable that the
production line is simulated under a constraint that the designated
process unit is stopped operating during a selected available checking
time period while sequentially selecting the available checking time
periods until the simulated output obtained by this step is not smaller
than the predetermined target figure.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic block diagram showing an embodiment of an inspection
scheduler according to the present invention;
FIG. 2 is a flowchart showing an embodiment of an inspection schedule
decision method according to the present invention;
FIG. 3 is a waveform diagram showing an example of the time-varying number
of in-process workpieces in the embodiment; and
FIG. 4 is a schematic diagram showing an example of simulation result to
explain an operation of the embodiment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIG. 1, there is shown a periodic-inspection scheduler which
uses a simulation technique to decide on an inspection schedule of process
units arranged in series forming a production line. In the
periodic-inspection scheduler, a processor 101 such as a CPU runs a
periodic-inspection schedule decision program stored in a memory 102 while
using a production line simulator 103, as will be described later.
Needless to say, a simulation program of the production line simulator 103
may be incorporated into the periodic-inspection schedule decision program
stored in the memory 102.
An input device 104 is used to input production line data required for the
simulation and a monitor 105 displays the result of a periodic-inspection
schedule decided by the program on screen. The production line data
includes a maximum capacity and a processing time of each process unit,
the processing sequence of the process units, the number of workpieces to
be processed, the number of in-process workpieces in each process unit,
and a target output of the production line.
The descriptions of first and second embodiments of the inspection-schedule
decision method according to the present invention will be described
hereinafter referring to FIGS. 2-4.
FIRST EMBODIMENT
Referring to FIG. 2, the above production line data together with a
simulation period Ts and designation data for designating at least one
process unit to be checked are input by the managing operator through the
input device 104 (step S201). When receiving the above data, the processor
101 starts the production line simulator 103 performing the simulation
using the production line data in the designated simulation period Ts
(step S202).
The simulator 103 returns the time-varying number Np of in-process
workpieces in each process unit and an output N.sub.RESULT of the
simulated production line to the processor 101. Based on the time-varying
number Np of in-process workpieces in each process unit, the processor 101
extracts a time period Tc during which the time-varying number Np is not
greater than a predetermined value Nb which is X% of an average of number
of workpieces which would be processed by each process unit to be checked
(step S203). For example, the value Nb is set to 10% of an average of
number of processed workpieces a day in each process unit.
In the first embodiment, a single process unit is designated as a process
unit to be checked. As a matter of practicality, it is useful to decide
the checking period of a single process unit in the production line. More
specifically, in the case where the number Np of in-process workpieces in
a single process unit varies with time as shown in FIG. 3, the processor
101 compares Np with Nb and, in this case, extracts two time periods
Tc.sub.1 and Tc.sub.2 within the simulation period Ts. There may be cases
where such a time period is not found. These time periods Tc.sub.1 and
Tc.sub.2 are used as available checking periods or candidate periods in
the following steps.
If there is at lease one check-available period within the simulation
period Ts (YES in step S204), one available checking period Tc is selected
and then it is determined whether the available checking period Tc is
equal to or longer than the checking time T.sub.CHECK required to check
that process unit (step S205). When Tc.gtoreq.T.sub.CHECK (YES in step
S205), the checking period of the process unit is set to the available
checking period Tc (step S206) and then the simulation is performed under
that constraint (step S207). In other words, under the constraint that the
process unit is stopped operating during the check time T.sub.CHECK within
the available checking period Tc, the simulation of the production line is
performed again by the simulator 103.
The simulator 103 returns a simulated output N.sub.RESULT of the production
line to the processor 101. If the simulated output N.sub.RESULT is equal
to or greater than the target output N.sub.TARGET (YES in step S208), the
check time T.sub.CHECK of the process unit is determined within the
available checking period Tc and is displayed on the monitor 105 (step
S209).
On the other hand, when Tc<T.sub.CHECK (NO in step S205) or when
N.sub.RESULT <N.sub.TARGET (NO in step S208), a subsequent available
checking period is selected (step S210) and then control goes back to the
step S204. Further, if there is no available checking period within the
simulation period Ts (NO in step S204), the simulation period Ts is
extended (step S211) and the simulation is performed again for the
extended simulation period Ts.
In FIG. 3, for example, if the time period Tc.sub.1 is shorter than
T.sub.CHECK or if N.sub.RESULT <N.sub.TARGET for the time period Tc.sub.1,
another time period Tc.sub.2 is selected and the above steps S204-S209 are
performed for the time period Tc.sub.2. Further, since the available
checking period Tc is selected so that the time-varying number Np is
smaller than the predetermined value Nb which is 10% of an average number
of workpieces which would be processed by the process unit (step S203),
the effect of the inspection on an output of the production line would be
small. Especially, since the time period TC.sub.2 provides a lower Np than
the time period Tc.sub.1, a better balance of the number of in-process
workpieces and a better output would be obtained, resulting in reliable
and stable operation of the production line.
SECOND EMBODIMENT
As shown in FIG. 4, an available checking period Tc may be obtained for
each of a plurality of process units to be checked. It is assumed for
simplicity that two process units P.sub.i and P.sub.j of the process units
P.sub.1 -P.sub.N are designated as a process unit to be checked in
simulation.
In the second embodiment, when receiving the respective time-varying
numbers N.sub.i and N.sub.j of in-process workpieces in the process units
P.sub.i and P.sub.j from the simulator 103, the processor 101 compares
Np.sub.i with Nb.sub.i and extracts time periods Tc.sub.i1 and Tc.sub.i2
as described before. Similarly, the processor 101 compares Np.sub.j with
Nb.sub.j and extracts a time period Tc.sub.j1. Therefore, if these time
periods satisfy the conditions of steps S204 and S205 of FIG. 2, two
checking schedule candidates are obtained, that is, a first checking
schedule candidate of Tc.sub.i1 and Tc.sub.j1 and a second checking
schedule candidate of Tc.sub.i2 and Tc.sub.j1.
Subsequently, the processor 101 performs the simulation under the
constraint of the first checking schedule candidate of Tc.sub.i1 and
Tc.sub.j1. In other words, under the constraint that the process units
P.sub.i and P.sub.j are stopped operating during the check time
T.sub.CHECKi within the check-available period Tc.sub.i1 and the check
time T.sub.CHECKj within the check-available period Tc.sub.j1,
respectively, the simulation of the production line is performed again by
the simulator 103. If the simulated output N.sub.RESULT is equal to or
greater than the target output N.sub.TARGET, the first checking schedule
of T.sub.CHECKi and T.sub.CHECKj is determined.
On the other hand, if the simulated output N.sub.RESULT is smaller than the
target output N.sub.TARGET, the second checking schedule candidate of
Tc.sub.i2 and Tc.sub.j1 is selected, and then the simulation under that
constraint is performed. When there is no checking schedule candidate, the
simulation period Ts is extended and the above steps will be repeated
until the simulated output N.sub.RESULT is equal to or greater than the
target output N.sub.TARGET.
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